1. Field of Invention
This invention relates to a method and apparatus for controlling the air-fuel ratio demanded by a fuel controller in order to maintain optimum performance of a catalytic converter where there is no feed back between the achieved air fuel ration and the desired air fuel ratio.
2. Related Art
Three way catalytic converters are used to reduce exhaust emission of nitrous oxides (NOx) hydrocarbon (HC) and carbon monoxide (CO). In a steady state of operation the performance of the catalyst in removing these pollutants is at an optimum level when the air fuel ratio of the exhaust gas is within a narrow range, close to the stoichiometric air-fuel ratio.
Conventionally, a fuel controller is used to control the air fuel ratio demand from an engine based on feedback from an air fuel ratio sensor upstream of a catalytic converter in the exhaust passage or from two air fuel ratio sensors, one upstream of the catalytic converter, and one downstream of the catalytic converter. Operation based on such feedback is known as ‘closed-loop’ operation.
However, when it is desired to operate the engine away from the stoichiometric, as the air fuel sensors are insensitive to changes in the air fuel ratio other than around the stoichiometric air fuel ratio, there is no feed back correction relating to the air-fuel ratio actually achieved. Such operation is known as ‘open-loop’ operation.
For example, after throttle opening it may be desirable to operate on the rich side of a stoichiometric air fuel ratio in order to provide extra torque. Another example is after deceleration fuel shut off it may be desirable to operate on the rich side of a stoichiometric air fuel ration in order to regenerate the catalyst.
The problem therefore is that when an engine is operating away from stoichiometric it is in open loop operation and any error between the requested air fuel ratio and the achieved air fuel ratio is not corrected.
The object of this invention is to provide a method and apparatus for correcting air fuel errors when in open-loop operation when the engine is operating with a substantially different air fuel ratio to that which is stoichiometric.